A highly sensitive localized surface plasmon resonance sensor chip is known (International Publication No. WO 2011/121857). The chip described in International Publication No. WO 2011/121857 has higher sensitivity than conventional ones. However, since an absorption spectrum originating from localized plasmon resonance has a relatively broad linewidth, some contrivance is required for this chip to detect minute spectral shifts. For this reason, in some cases, the absorption spectrum of a chip is accurately measured by a spectral photometer having high wavelength resolution, and a spectral shift before and after sensing is analyzed based on a technique such as peak fitting. It is very difficult to measure spectra by using conventional tabletop-size spectral photometers at a bedside, a clinical site, an airport and a seaport, or a region without a sufficient social infrastructure. There has been a demand for the advent of a sensing chip and a sensing system which can accurately and easily measure the peak wavelength shift of an optical spectrum in any places.